The present invention relates to computer control apparatus. More particularly, it relates to a multiplexer circuit for a process control computer system.
In the art of process control apparatus, the primary sensors are usually analog devices which respond to process variable conditions, such as pressure, temperature, flow, volume and the like, to produce an analog signal. The analog signal is then applied to the control system from which is derived a control signal to control the process at desired levels. In analog control systems, the input analog signals were, indeed, handled as analog signals by an analog controller. In some cases, by a time sharing technique, one analog controller could control several data points. In large instrumentation systems, such analog control becomes very expensive and complex. Digital control systems provide a means for handling a large number of control functions by a single digital computer. Such a system is shown in copending application of Woods et al, Ser. No. 773,913, filed Mar. 3, 1977.
In order for the digital computer to handle the signals from a great number of input sensors, first, multiplexing means are provided for selecting one at a time of the numerous input circuits for handling. Second, means are provided for selectively converting the analog signals from the sensors into equivalent digital signals for presentation to the digital computer.